Marine natural products such as marine algae, fish and shellfish that live in the sea generally take up iodine either directly from the sea water or indirectly through ingestion and accumulate it in their bodies. Among these marine natural products, marine algae are known to contain a lot of iodine. For example, kombu (edible kelp) belonging to brown algae is known to contain a high concentration of iodine, as much as about 500 to 60000 times higher than the iodine content of the sea water. Due to this fact, marine natural product extracts obtained by extraction using, as a raw material, marine natural products such as kombu contain a high concentration of iodine.
Marine natural products as described above and marine natural product extracts obtained using these marine natural products as a raw material have been very commonly eaten in Japan. For example, kombu and a kombu extract obtained using kombu as a raw material are often used to make soup stock and also used in a wide variety of condiments and processed foods.
Iodine is an essential element for the human body. However, in cases where an excessive amount of iodine is ingested, there is a potential risk of development of symptoms such as hypothyroidism and goiter. Accordingly, the Ministry of Health, Labour and Welfare of Japan has established the estimated requirement and the tolerable upper intake level in “Dietary Reference Intakes for Japanese”. Hence, in recent years, demands have been raised for a method for reducing iodine contained in marine natural product extracts and marine natural product extracts having a reduced iodine content.
Conventional processing methods for separating iodine ions from an aqueous solution etc. include a processing method for separating iodine ions from an aqueous solution by double substitution electrodialysis, the method being characterized in that chlorine ions are employed as an counter anion for iodine ions and that a monovalent anion permselective membrane is used as an anion exchange membrane (for example, see Patent Literature 1). Another example of such conventional methods is a method for removing iodine from a salt water containing Na, Cl, and I, the method being characterized in that iodine in the salt water is removed by means of an ion exchange resin while the oxidation reduction potential of the salt water is maintained at 100 mV (SHE) or less (for example, see Patent Literature 2). Another proposed method is a processing method for extracting fucoidan from a marine alga in the presence of a reducing substance (for example, see Patent Literature 3).
However, in the above double substitution electrodialysis, components (salt etc.) other than iodine contained in a marine natural product extract are also removed simultaneously. As a result, the obtained marine natural product extract shows imbalance in the component composition as compared with the original marine natural product extract before processing. In the above method with the use of an ion exchange resin, distinctive flavor components contained in a marine natural product extract are also adsorbed to the resin, which results in significant deterioration of the flavor. In the processing method for extracting from a marine alga an extract having a reduced iodine content in the presence of a reducing substance, the addition of the reducing substance greatly changes the flavor of a marine natural product extract.
Therefore, a need has arisen for a processing method of a marine natural product extract, the method being capable of producing a marine natural product extract with excellent flavor by removing iodine without losing taste components and umami components, such as salts and balanced amino acids, contained in the marine natural product extract.